My neighbor has a hot water tank that quit working earlier this week. I checked the breaker - looked good. I measured the voltage at the hot water tank; it read ~15Vac. I checked the double breaker for the hot water tank and it also measured ~15Vac. One step further, the line-to-line voltage coming into the main breaker read ~15Vac. Both 120Vac L-N branches give a voltage but one is at ~118Vac (good) and the other is around ~105Vac (hence my ~15Vac differential, it appears on the surface). When I open the main breaker, both L-N at the main breaker primary read ~120Vac. The hot water tank is the only 208/220 circuit in the house (and the only double breaker). Any idea what could be causing this suddenly? It looked to me like both branches were being fed, in phase, and that is why the difference between them is literally the difference between them. I had read that an open circuit on one of the main lines could cause this (the open line being backfed by the good line through the double breaker). But I'm not sure why the hot water tank would have worked up until this point.
I am not sure if this all adds up but maybe you lost a leg, and one leg is backfeeding the other through the water heater, you're seeing voltage drop from 118 to 105 across the water heater. (I'd have guessed a bigger voltage drop but still...)
With the water heater breaker off, do you lose power anywhere?
Probably the two phases are taken from same 'tap' at the transformer (or at the meter), one leg is more loaded than the other hence the 15V difference between the two.
If system used to work and suddently had problem, probably an utility's tecnichian miswired something during a maintanance.
My suggestion is to call your utility and have it fixed: tell them you have 15V difference between the legs.